DESCRIPTION

The sglist API manages physical address ranges. Each list contains one
or more elements. Each element contains a starting physical address and
a length. Scatter/gather lists are read-only while they are shared. If
one wishes to alter an existing scatter/gather list and does not hold the
sole reference to the list, then one should create a new list instead of
modifying the existing list.
Each scatter/gather list object contains a reference count. New lists
are created with a single reference. New references are obtained by
calling sglist_hold and are released by calling sglist_free.
AllocatingandInitializingLists
Each sglist object consists of a header structure and a variable-length
array of scatter/gather list elements. The sglist_alloc function
allocates a new list that contains a header and nsegs scatter/gather list
elements. The mflags argument can be set to either M_NOWAIT or M_WAITOK.
The sglist_count function returns the number of scatter/gather list
elements needed to describe the physical address ranges mapped by a
single kernel virtual address range. The kernel virtual address range
starts at buf and is len bytes long.
The sglist_build function allocates a new scatter/gather list object that
describes the physical address ranges mapped by a single kernel virtual
address range. The kernel virtual address range starts at buf and is len
bytes long. The mflags argument can be set to either M_NOWAIT or
M_WAITOK.
The sglist_clone function returns a copy of an exising scatter/gather
list object sg. The mflags argument can be set to either M_NOWAIT or
M_WAITOK. This can be used to obtain a private copy of a scatter/gather
list before modifying it.
The sglist_init function initializes a scatter/gather list header. The
header is pointed to by sg and is initialized to manage an array of
maxsegs scatter/gather list elements pointed to by segs. This can be
used to initialize a scatter/gather list header whose storage is not
provided by sglist_alloc. In that case, the caller should not call
sglist_free to release its own reference and is responsible for ensuring
all other references to the list are dropped before it releases the
storage for sg and segs.
ConstructingScatter/GatherLists
The sglist API provides several routines for building a scatter/gather
list to describe one or more objects. Specifically, the sglist_append
family of routines can be used to append the physical address ranges
described by an object to the end of a scatter/gather list. All of these
routines return 0 on success or an error on failure. If a request to
append an address range to a scatter/gather list fails, the
scatter/gather list will remain unchanged.
The sglist_append function appends the physical address ranges described
by a single kernel virtual address range to the scatter/gather list sg.
The kernel virtual address range starts at buf and is len bytes long.
The sglist_append_mbuf function appends the physical address ranges
described by an entire mbuf chain m to the scatter/gather list sg.
The sglist_append_phys function appends a single physical address range
to the scatter/gather list sg. The physical address range starts at
paddr and is len bytes long.
The sglist_append_uio function appends the physical address ranges
described by a uio(9) object to the scatter/gather list sg. Note that it
is the caller’s responsibility to ensure that the pages backing the I/O
request are wired for the lifetime of sg. Note also that this routine
does not modify uio.
The sglist_append_user function appends the physical address ranges
described by a single user virtual address range to the scatter/gather
list sg. The user virtual address range is relative to the address space
of the thread td. It starts at buf and is len bytes long. Note that it
is the caller’s responsibility to ensure that the pages backing the user
buffer are wired for the lifetime of sg.
The sglist_consume_uio function is a variation of sglist_append_uio. As
with sglist_append_uio, it appends the physical address ranges described
by uio to the scatter/gather list sg. Unlike sglist_append_uio, however,
sglist_consume_uio modifies the I/O request to indicate that the appended
address ranges have been processed similar to calling uiomove(9). This
routine will only append ranges that describe up to resid total bytes in
length. If the available segments in the scatter/gather list are
exhausted before resid bytes are processed, then the uio structure will
be updated to reflect the actual number of bytes processed, and
sglist_consume_io will return zero to indicate success. In effect, this
function will perform partial reads or writes. The caller can compare
the uio_resid member of uio before and after calling sglist_consume_uio
to determine the actual number of bytes processed.
ManipulatingScatter/GatherLists
The sglist_join function appends physical address ranges from the
scatter/gather list second onto first and then resets second to an empty
list. It returns zero on success or an error on failure.
The sglist_split function splits an existing scatter/gather list into two
lists. The first length bytes described by the list original are moved
to a new list *head. If original describes a total address range that is
smaller than length bytes, then all of the address ranges will be moved
to the new list at *head and original will be an empty list. The caller
may supply an existing scatter/gather list in *head. If so, the list
must be empty. Otherwise, the caller may set *head to NULL in which case
a new scatter/gather list will be allocated. In that case, mflags may be
set to either M_NOWAIT or M_WAITOK. Note that since the original list is
modified by this call, it must be a private list with no other
references. The sglist_split function returns zero on success or an
error on failure.
The sglist_slice function generates a new scatter/gather list from a sub-
range of an existing scatter/gather list original. The sub-range to
extract is specified by the offset and length parameters. The new
scatter/gather list is stored in *slice. As with head for sglist_join,
the caller may either provide an empty scatter/gather list, or it may set
*slice to NULL in which case sglist_slice will allocate a new list
subject to mflags. Unlike sglist_split, sglist_slice does not modify
original and does not require it to be a private list. The sglist_split
function returns zero on success or an error on failure.
MiscellaneousRoutines
The sglist_reset function clears the scatter/gather list sg so that it no
longer maps any address ranges. This can allow reuse of a single
scatter/gather list object for multiple requests.
The sglist_length function returns the total length of the physical
address ranges described by the scatter/gather list sg.

RETURNVALUES

The sglist_alloc, sglist_build, and sglist_clone functions return a new
scatter/gather list on success or NULL on failure.
The sglist_append family of functions and the sglist_consume_uio,
sglist_join, sglist_slice, and sglist_split functions return zero on
success or an error on failure.
The sglist_count function returns a count of scatter/gather list
elements.
The sglist_length function returns a count of address space described by
a scatter/gather list in bytes.

ERRORS

The sglist_append functions return the following errors on failure:
[EINVAL] The scatter/gather list has zero segments.
[EFBIG] There are not enough available segments in the
scatter/gather list to append the specified physical
address ranges.
The sglist_consume_uio function returns the following error on failure:
[EINVAL] The scatter/gather list has zero segments.
The sglist_join function returns the following error on failure:
[EFBIG] There are not enough available segments in the
scatter/gather list first to append the physical
address ranges from second.
The sglist_slice function returns the following errors on failure:
[EINVAL] The original scatter/gather list does not describe
enough address space to cover the requested sub-range.
[EINVAL] The caller-supplied scatter/gather list in *slice is
not empty.
[ENOMEM] An attempt to allocate a new scatter/gather list with
M_NOWAIT set in mflags failed.
[EFBIG] There are not enough available segments in the caller-
supplied scatter/gather list in *slice to describe the
requested physical address ranges.
The sglist_split function returns the following errors on failure:
[EDOOFUS] The original scatter/gather list has more than one
reference.
[EINVAL] The caller-supplied scatter/gather list in *head is
not empty.
[ENOMEM] An attempt to allocate a new scatter/gather list with
M_NOWAIT set in mflags failed.
[EFBIG] There are not enough available segments in the caller-
supplied scatter/gather list in *head to describe the
requested physical address ranges.